Spin Transition Sensors Based on β-Amino-Acid 1,2,4-Triazole Derivative

A β-aminoacid ester was successfully derivatized to yield to 4H-1,2-4-triazol-4-yl-propionate (βAlatrz) which served as a neutral bidentate ligand in the 1D coordination polymer [Fe(βAlatrz)3](CF3SO3)2·0.5H2O (1·0.5H2O). The temperature dependence of the high-spin molar fraction derived from 57Fe Mossbauer spectroscopy recorded on cooling below room temperature reveals an exceptionally abrupt single step transition between high-spin and low-spin states with a hysteresis loop of width 4 K (Tc↑ = 232 K and Tc↓ = 228 K) in agreement with magnetic susceptibility measurements. The material presents striking reversible thermochromism from white, at room temperature, to pink on quench cooling to liquid nitrogen, and acts as an alert towards temperature variations. The phase transition is of first order, as determined by differential scanning calorimetry, with transition temperatures matching the ones determined by SQUID and Mössbauer spectroscopy. The freshly prepared sample of 1·0.5H2O, dried in air, was subjected to annealing at 390 K, and the obtained white compound [Fe(βAlatrz)3](CF3SO3)2 (1) was found to exhibit a similar spin transition curve however much temperature was increased by (Tc↑ = 252 K and Tc↓ = 248 K). The removal of lattice water molecules from 1·0.5H2O is not accompanied by a change of the morphology and of the space group, and the chain character is preserved. However, an internal pressure effect stabilizing the low-spin state is evidenced

Iron(II) spin transition coordination polymers with a zigzag structure

The synthesis and characterisation of seven iron(ii) 1D chain coordination polymers with tetradentate Schiff-base like equatorial ligands and bis(4-pyridylmethyl)sulfide (bpms) as a flexible bridging axial ligand is reported. This new family of materials displays a wide spectrum of spin transition properties in the solid state ranging from gradual, abrupt, incomplete to even step-wise that have all been characterized by SQUID magnetometry. The X-ray structure analysis of two complexes at several temperatures is discussed in the frame of their spin crossover properties. © 2012 The Royal Society of Chemistry

Pressure and Temperature Spin Crossover Sensors with Optical Detection

Iron(II) spin crossover molecular materials are made of coordination centres switchable between two states by temperature, pressure or a visible light irradiation. The relevant macroscopic parameter which monitors the magnetic state of a given solid is the high-spin (HS) fraction denoted nHS, i.e., the relative population of HS molecules. Each spin crossover material is distinguished by a transition temperature T1/2 where 50% of active molecules have switched to the low-spin (LS) state. In strongly interacting systems, the thermal spin switching occurs abruptly at T1/2. Applying pressure induces a shift from HS to LS states, which is the direct consequence of the lower volume for the LS molecule. Each material has thus a well defined pressure value P1/2. In both cases the spin state change is easily detectable by optical means thanks to a thermo/piezochromic effect that is often encountered in these materials. In this contribution, we discuss potential use of spin crossover molecular materials as temperature and pressure sensors with optical detection. The ones presenting smooth transitions behaviour, which have not been seriously considered for any application, are spotlighted as potential sensors which should stimulate a large interest on this well investigated class of materials

Size Effect and Role of Short- and Long-Range Interactions on 1D Spin-Crossover Systems within the Framework of an Ising-Like Model

This contribution, motivated by an experimental study of the influence of long-range interactions on the hysteresis loop, presents a theoretical study of spin-state transitions within the framework of an Ising-like model. The Hamiltonian includes both short- and long-range interactions and takes into account different degeneracies between molecular states. The problem is solved exactly for one-dimensional systems by using a transfer matrix method and the effects of temperature, long- and short-range interactions, and system size (number of molecules) are analysed in depth. The width of the thermal hysteresis loop of spin-crossover compounds decreases when reducing the long-range interactions, down to a critical value, calculated for the first time in this contribution, at which the hysteresis vanishes. This dependence is similar to the one generated by the decrease in short-range interactions assuming that the corresponding variation is proportional. An increase in the system size contributes to a larger influence of long-range interactions and the hysteretic loop approaches a rectangular shape and reaches a saturation width, which is predicted for the first time. These results are important for the design of novel spin-crossover materials featuring sharp spin transitions along with wide hysteresis loops, which are particularly suited for potential applications in display and data storage devices

Effect of texture alteration by thin film fabrication on the spin crossover of [Fe(3-Br-phen)/sub 2/(NCS)/sub 2/] middot 0.5CH/sub 3/OH

A mononuclear complex of composition [Fe(3-Br-phen)/sub 2/(NCS)/sub 2/] middot 0.5CH/sub 3/ OH which exhibits a gradual, incomplete spin crossover (SCO) in bulk sample (1) was selected to study the effect of alteration of particle size and shape on SCO properties. Thin films (2) were fabricated using spin coating technique on quartz substrates and silicon wafers. Morphology analysis by scanning electron micrographs shows a drastic reduction in particles size and deformation in shape compared to the bulk sample. Nearly spherical particles of 200-500 nm diameter were obtained and further reduction in size down to 50-120 nm were obtained with increase in spin coating speed. A thin film on a quartz substrate was studied by /sup 57/Fe Mössbauer spectroscopy in transmission mode which shows only a low-spin signal as opposed to a dominant high-spin signal in bulk sample thus hinting out the effect of texture modification on increasing the ligand field strength.Anglai

Neutral and anionic duality of 1,2,4-triazole α-amino acid scaffold in 1D coordination polymers

A tiny supramolecular synthon, 4H-1,2,4-triazol-4-yl acetic acid (HGlytrz) which is bifunctional by design having an electronic asymmetry and conformational flexibility has been introduced to synthesize iron(II) complexes. Having 1,2,4-triazole or carboxylic extremities on the same framework HGlytrz could display dual functionality by acting as a neutral as well as anionic ligand based on the possibility of deprotonation of carboxylic group. Four new iron(II) HGlytrz complexes with ClO 4 - (1), NO 3 - (2), BF 4 - (3) and CF 3SO 3 - (4) anions were prepared. Formulation of their composition which is complicated due to ligand deprotonation is discussed. Unlike its ester protected counterpart ethyl-4H-1,2,4-triazol-4-yl-acetate (αGlytrz) which show hysteretic room temperature spin crossover, 1-4 remain in the high-spin state as revealed by 57Mössbauer spectroscopy. Prospects of such 1D coordination polymers with dangling unbounded carboxylic entities in the realm of self-assembled monolayer (SAM) are discussed. © 2011 Springer Science+Business Media B.V